Rains may pound the Central Valley and Bay Area relentlessly but it won’t help much if rain and heavy snow doesn’t fall in the Sierra.
Over 60 percent of the water Californians rely on for urban and farm uses originates from the Sierra, mostly in the form of snowpack.
Thursday’s storm that drenched the Northern San Joaquin Valley dumped significantly less rain in the foothills and didn’t bury the Southern Sierra in snow as one might expect. That’s because it was a storm system known as an atmospheric river that on the West Coast has the moniker “Pineapple Express” since it collects and transports moisture from near Hawaii.
The storm during a 24-hour period on Thursday dumped 3.36 inches in Ripon, 1.98 inches in Manteca, 0.96 inches in Lathrop, 2.50 inches in Oakdale, 2.91 inches in Copperopolis and 1.09 inches in Sonora.
But in the New Melones/Stanislaus River watershed it dropped 0.93 inches at Arnold, 0.24 inches at Strawberry, and 0.16 inches above the highest two Tri-Dam Project reservoirs South San Joaquin Irrigation District operates in conjunction with Oakdale Irrigation District.
“It is helping recharge underground water tables but didn’t do a heck of a lot for the Sierra reservoirs,” noted SSJID General Manager Jeff Shields.
And since 60 percent of the state’s water critical to cities and agriculture is derived from the Sierra, Thursday’s major storm didn’t help as much as some hoped to reduce the water deficit in the Southern Sierra caused by three straight years of drought.
Data compiled by SSJID Finance and Administration Manager Bere Lindley using National Weather Service rainfall readings noted rainfall and snow as you head north — starting with the Mokelumne River watershed — was significantly higher. There was 4.55 inches of rain at San Andreas. 4.92 inches at Pine Grove, and 2.27 inches at Wisley.
Going south of the Stanislaus River watershed that flows into New Melones, rainfall readings were 0.29 inches at Groveland and 0.21 inches just southwest of the 120 entrance to Yosemite National Park. That water flows into the Tuolumne River and Don Pedro Reservoir.
In the state’s far north, the watershed feeding Shasta Reservoir — California’s largest reservoir — had some of the heaviest snow from the storm plus rain totals approaching that of the Northern and Joaquin Valley.
The atmospheric river dumped most of its precipitation in the Central Valley and Bay Area before it played out and moved east of the foothills where relatively unspectacular amounts of snow fell.
Atmospheric rivers usually strike the Central Valley once a year but usually don’t have n above average volume of moisture. Every two years or so typically a larger event occurs. The last major such storm was in October 2008.
Atmospheric rivers are relatively narrow regions in the atmosphere that are responsible for most of the horizontal transport of water vapor outside of the tropics. While they come in many shapes and sizes, those that contain the largest amounts of water vapor, the strongest winds, and stall over watersheds vulnerable to flooding, can create extreme rainfall and floods. They are also responsible for 30 to 50 percent of the annual precipitation in West Coast states
Thursday’s event was 400 miles wide.
An atmospheric river or Pineapple Express slammed California in December of 1996 but instead of stalling as the one did last week it drifted eastward delivering heavy rainfall in the elevations triggering an unusually early and major snow melt just before New Year’s Day. Snowfall also happened to be significantly above normal for December. It is what triggered major flooding south of Manteca when levees failed flooding 70 square miles and causing $100 million in damage.
Drought still has
state in its grip
It is part of the reason state water officials are urging people to continue to conserve water. Experts say it will take at least 150 percent of normal precipitation to ease the drought.
Shields noted as things stand right now, there will be at least 600,000 acre feet of inflow into New Melones reservoir. In exchange for allowing the Bureau of Reclamation to flood the original Melones Dam built by SSJID and OID in 1925, each water agency splits the first 600,000 acre feet of water that flows in to the reservoir.
For the water year that ended Sept. 30, only 346,000 acre feet flowed into New Melones. That meant the OID and SSSJID were the only ones that took water deliveries. They were able to get by with tight conservation measures and carryover storage.
The reservoir beholds 1.1 million acre feet of water.
“At 70 percent of normal we’re fine,” Shields said of snowfall.
An estimated 68 percent of the water used in cities and on farms each year comes from the Sierra snowpack.
That said Shields made it clear the SSJID is not going to ease up one drop on water conservation.
“It is still bad for the rest of California,” Shields said. “Even if we get 100 percent of precipitation it’s bad for the state as a whole.”
He noted many water districts desperate to keep water flowing to crops and through household taps are already looking to SSJID and OID to sell weather in 2015 that they can spare.